1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for controlling cell load in a wireless communication system.
2. Discussion of the Related Art
Generally, WCDMA (wideband code division multiple access) based 3GPP (3rd generation partnership project) wireless communication systems are ongoing to be widely spread over the world. WCDMA system has started from Release 99 (R99) and had introduced HSPDA (high speed downlink packet access) and HSUPA (high speed uplink packet access) as wireless access technologies having high competitiveness in mid-term future. The WCDMA system also introduces E-UMTS as a wireless access technology having high competitiveness in long-term future. The E-IMTS is the system that has evolved from WCDMA UMTS and its standardization is ongoing by 3GPP. Moreover, the E-UMTS is called LTE (long term evolution) system. For the details of technical specifications of UMTS and E-UMTS, it is able to refer to Release 7 and Release 8 of ‘3rd Generation Partnership Project Technical Specification Group Radio Access Network’, respectively.
FIG. 1 is a diagram of a network structure of UMTS (universal mobile telecommunications system).
Referring to FIG. 1, a UMTS includes a user equipment (hereinafter abbreviated UE), a UMTS radio access network (hereinafter abbreviated UTRAN) and a core network (hereinafter abbreviated CN). The UTRAN includes at least one or more radio network subsystems (RNS). Each of the radio network subsystems (RNS) includes a single radio network controller (hereinafter abbreviated RNC) and at least one base station (Node B). The RNC manages the at least one base station. And, at least one or more cells exist in one base station.
FIG. 2 is a diagram of a radio protocol used for UMTS.
Referring to FIG. 2, radio protocol layers exist as a pair in a user equipment and a UTRAN and are responsible for data transfer in a radio interface. All protocols are inserted in one UE, whereas protocols can be distributed per network element in UTRAN. Comparing to the generally-known OSI (open systems interconnection) reference model, a physical layer (PHY) corresponds to a first layer (L1). MAC (medium access control), RLC (radio link control), PDCP (packet data convergence protocol) and BMC (broadcast/multicast control) layers correspond to a second layer (L2), respectively. And, RRC (radio resource control) layer corresponds to a third layer (L3). Information exchange between protocol layers is performed via a virtual access point called a service access point (hereinafter abbreviated SAP).
The PHY layer plays a role in transferring data in a radio interface using various radio transmission technologies. The PHY layer is responsible for a reliable data transfer in a radio interface. Data multiplexing, channel coding, spreading, modulation and the like are applied to the PHY layer for the data transfer. The PHY layer is connected to the MAC layer, which is an upper layer, via a transport channel. The transport channel can be classified into a dedicated transport channel or a common transport channel according to a presence or non-presence of channel sharing.
The MAC layer plays a role in mapping various logical channels to various transport channels. And, the MAC layer also plays a role as logical channel multiplexing for mapping several logical channels to one transport channel. The MAC layer is connected to an upper layer (RLC layer) via a logical channel. The logical channel can be classified into a control channel for carrying information of a control plane or a traffic channel for carrying information of a user plane according to a type of transmitted information.
The MAC layer can be divided into a MAC-b sublayer, a MAC-d sublayer, a MAC-c/sh sublayer, a MAC-hs/ehs sublayer and a MAC-e/es or a MAC-/i/is sublayer. First of all, the MAC-b sublayer is responsible for management of BCH (broadcast channel) that is a transport channel responsible for broadcasting of system information. The MAC-c/sh sublayer manages such a common transport channel shared with other user equipments as a FACH (forward access channel) and a DSCH (downlink shared channel). The MAC-d sublayer is responsible for management of a DCH (dedicated channel) that is a dedicated transport channel for a specific user equipment. To support high speed downlnk data transfer, the MAC-hs/ehs sublayer manages a HS-DSCH (high speed downlink shared channel) that is a transport channel for high speed downlink data transfer. And, the MAC-e/es or MAC-/i/is sublayer manages a E-DCH (enhanced dedicated channel) that is a transport channel for high speed uplink data transfer.
The RLC layer is responsible for QoS guarantee of a radio bearer (hereinafter abbreviated RB) and corresponding data transfer. The RLC layer comprises one or two independent RLC entities for each RB to guarantee unique QoS of the corresponding RB. The RLC layer provides three kinds of RLC modes, which include transparent mode (TM), unacknowledged mode (UM) and acknowledged mode (AM), to support various QoS. Besides, the RLC layer plays a role in adjusting a data size to enable a lower layer to fit for a transfer data in a radio interface. For this, the RLC layer performs functions of segmentation and/or concatenation on data received from an upper layer.
The PDCP layer is located above the RLC layer. The PDCP layer enables data, which is carried on such an IP packet as IPv4 and IPv6, to be efficiently transferred in a radio interface having a relatively narrow bandwidth. For this, the PDCP layer performs a header compression function. In this case, the header compression function enables a header part of data to carry essential information only, thereby increasing transfer efficiency in a radio interface. As the header compression is a basic function of the PDCP layer, the PDCP layer mainly exists in a packet switched (PS) domain. In order to provide a header compression function effective for a PS service, one PDCP entity exists per RB. On the contrary, in case that the PDCP layer exists in CS domain, the header compression function is not provided.
The BMC layer exists above the RLC layer. The BMC layer performs a function of scheduling a cell broadcast message and a function of broadcasting a cell broadcast message to user equipments existing in a specific cell.
The RRC layer is located at a bottom of the third layer and is defined in a control plane only. In association with configuration, reconfiguration and release of radio bearers (RBs), the RRC layer controls parameters of the first and second layers. And, the RRC layer controls logical channels, transport channels and physical channels. In this case, the RB means a logical path provided for data transfer between a user equipment and a UTRAN by the first and second layers of the radio protocol. Generally, the RB configuration means a process for specifying a radio protocol layer and channel characteristics required for providing a specific service and a process for setting detailed parameters and operating methods.
In a WCDMA system, an R99 UE, which is in a CELL_DCH state, transmits a large amount of data using a DCH and a Release 6 UE transmits a large amount of data using an E-DCH/HS-DSCH. The R99 UE transmits data using an RACH in an IDLE state or a CELL_FACH state. The CELL_FACH state is a state in which a dedicated physical channel has not been allocated to the UE and the UE uses a common transmission channel although an RRC connection has been established. However, in the CELL_FACH state, the UE can transmit data using a dedicated logical channel. The CELL_FACH state is generally used when the amount of traffic exchanged between the UE and the UTRAN is small. The UE receives data while monitoring an FACH and uses an RACH when transmitting data in uplink. When the UE is in the CELL_FACH state, the UE can receive a BCH in order to obtain system information. Since the UE which is in the CELL_FACH state uses a common transmission channel, RNTI information for UE identification may be included in a MAC header. Since data transmission is possible through the RACH although no RRC connection has been established, the RACH has a small signaling delay time, compared to the E-DCH. For HTTP transmission or keep alive message transmission, a common E-DCH, which achieves the advantages of both the RACH and the E-DCH such as rapid signaling and large data transmission capacity, is being discussed in an “Enhanced Uplink for CELL_FACH state for FDD” Work Item (WI).
FIG. 3 illustrates dedicated E-DCH transmission, legacy RACH transmission, and common E-DCH transmission. In FIG. 3, the horizontal axis represents time and the vertical axis represents power level.
First, a method for performing dedicated E-DCH transmission is described. The network knows E-RNTIs which are identities of UEs. The network can allocate wireless resources to each UE on a Transmission Time Interval (TTI) basis through an Enhanced Absolute Grant Channel (E-AGCH) according to wireless environments that change dynamically and rapidly. Accordingly, the UE can transmit data only when the network allocates resources to the UE every TTI. The E-AGCH is a common channel that is received by all UEs in the cell. Since a number of UEs in the cell receive the same E-AGCH, a Cyclic Redundancy Check (CRC) generated using an E-RNTI of a specific UE is used to allocate resources to the specific UE. Accordingly, CRC checking will be successful for each UE to which resources have been allocated and CRC checking will be unsuccessful for each UE to which resources have not been allocated. The E-AGCH message has a 5-bit Absolute Grant (AG) value and a 1-bit AG scope. The AG value indicates maximum E-DCH traffic, which can be used in the next TTI, using an E-DPDCH/DPCCH rate (see “Previous Mapping of Absolute Grant Value” in FIG. 13). The UE determines a serving grant and whether or not an HARQ is active using an E-AGCH. Thereafter, the UE determines the maximum number of bits for transmission from the serving grant using a serving grant update function and an Enhanced Transport Format Combination (E-TFC) selection function. Accordingly, the amount of data that can be transmitted through an E-DCH can be changed every TTI according to the maximum amount of E-DPDCH/DPCCH resources allocated by the network.
The following is a description of a legacy random access procedure (RACH). The RACH is used to transmit short-length data in uplink. Some RRC messages such as an RRC connection request message, a cell update message, and a URA update message are also transmitted through the RACH. Logical channels such as a Common Control Channel (CCCH), a Dedicated Control Channel (DCCH), or a Dedicated Traffic Channel (DTCH) can be mapped to a transmission channel (RACH) The transmission channel (RACH) can be mapped to a physical channel (e.g., a Physical Random Access Channel (PRACH)). When a UE Medium Access Control (MAC) layer instructs a UE physical layer to transmit a PRACH, the UE physical layer first selects an access slot and a signature and transmits a PRACH preamble in uplink. The preamble is transmitted during an access slot duration that is 1.33 ms long and a signature selected from among 16 signatures is transmitted during a predetermined initial period of the access slot. When the UE transmits a preamble, the Node B transmits a response signal through a downlink physical channel, i.e., an Acquisition Indicator Channel (AICH). The AICH transmitted as a response to the preamble carries the signature selected by the preamble during the first predetermined period of the access slot through which the preamble has been transmitted. The Node B transmits an acknowledgement (ACK) or a negative acknowledgement (NACK) through the signature transmitted through the AICH.
The following is a description of the method for transmitting a common E-DCH. The common E-DCH is being discussed as a WI but not all procedures thereof have been discussed. A common E-DCH that has been approved is described below. The common E-DCH transmission method is a combination of the dedicated E-DCH transmission method with the legacy RACH. A channel allocation request for data transmission is issued using the RACH preamble, i.e. L1 PRACH transmission, and data transmission is performed using the E-DCH. Since the channel that a UE which is in a CELL_FACH uses for uplink access is limited to the RACH, the UE requests an E-DCH using a random access procedure when there is a need to transmit a large amount of uplink data. Specifically, configuration information for the common E-DCH is transmitted to all UEs in the cell through a common E-DCH system Information Element (IE) of SIB 5 and SIB 5bis that the Node B broadcasts to the cell. The UE transmits a preamble for access to the common E-DCH channel in uplink and the Node B transmits an ACK or NACK to the UE through an AICH. When a common E-DCH has been allocated, the Node B transmits an AG value through an E-AGCH to allocate wireless resources to each UE. The UE monitors the E-AGCH until the common E-DCH is released. The Node B sets the AG value to “INACTIVE” to release the common E-DCH.
FIG. 4 is a flow chart illustrating an RACH transmission control procedure.
As shown in FIG. 4, a UE receives an RACH parameter from an RRC (S402). Thereafter, when data to be transmitted is present, the UE selects an Access Service Class (ASC) (S404 and S406) and attempts an L1 PRACH transmission procedure using a probability value obtained from the selected ASC (S408). When the UE performs L1 PRACH transmission, the UE attempts preamble ramping. That is, when preamble transmission has failed, the UE increases preamble transmission power by a ramping step when performing preamble retransmission. Thereafter, the UE monitors the AICH in order to check whether or not preamble transmission is successful (S410). When an ACK has been received, the UE assumes that L1 PRACH transmission is successful and transmits data in the next TTI. When no response (i.e., ACK) has been received, the UE reattempts the L1 PRACH transmission procedure after waiting 10 ms. On the other hand, when a NACK has been received, the UE reattempts the L1 PRACH transmission procedure after waiting a back-off time. The back-off time is broadcast to all UEs in the cell using an SIB. That is, the back-off time is a cell-specific parameter and is commonly applied to all UEs in the cell that attempt random access.
The following is a description of setup of a radio bearer according to QoS. QoS stands for “Quality of Service”, which an end user experiences when receiving a specific service. Typical factors that affect QoS include delay, error ratio, and bit rate. Specifically, examples of the delay include delay for channel allocation and delay for establishment of a connection when a number of users are present. When providing a service to an end user, first, the UMTS determines appropriate QoS according to the type of the service. Here, the appropriate QoS is the minimum QoS with which the end user can receive the service without difficulty. By setting the appropriate QoS to the minimum QoS, the UMTS can provide the service to a number of users. That is, if the UMTS provides a service with high QoS only to a specific user although wireless resources are limited, a large amount of wireless resources is allocated to the specific user, and therefore the total number of users to which the UMTS can provide services is reduced from the cell viewpoint.
For example, let us assume that a Serving GPRS Support Node (SGSN) has received a request for a Voice over Internet Protocol (VoIP) service from a UE. In this case, the SGSN determines QoS suitable for providing a VoIP service taking into consideration resources, priority, and capabilities of the UE. Thereafter, the SGSN notifies a Radio Network Controller (RNC) of the determined QoS-Uu. The RNC sets up RBs suitable for the respective QoSs of all UEs based on the QoS-Uu. Since a number of UEs requesting different QoSs are present in the cell, the network should perform load control and congestion control so as to satisfy all QoSs.
In the recent UMTS release 8, use of a common E-DCH is being discussed in an “Enhanced uplink for CELL_FACH state in FDD” Work Item (WI) in order to transmit services such as VoIP and HTTP as efficiently as possible. If random access is successful after a preamble is transmitted, then the legacy RACH transmits a message during only one TTI. However, if access is successful after a preamble is transmitted, then the E-DCH transmits data during a number of TTIs until the common E-DCH is released. For the network, it is necessary to perform load control of the common E-DCH since the common E-DCH transmits a large amount of data, unlike the legacy RACH. Particularly, there is a need to control uplink access load when a UE attempts to re-access a common E-DCH that has been released.